Liquid crystal electrooptic modulation has been utilized in a number of device applications. Nematic liquid crystals provide analog retardation changes due to rotation of birefringent molecules out of the plane of the incident optical field with tuning speeds of 1-100 ms. Chiral smectic liquid crystals (CSLCs) provide tuning speeds of 1 .mu.s. When incorporated in a "bookshelf" geometry cell (smectic layers oriented perpendicular to the substrate walls), analog CSLC materials, such as SmA.sup.* and distorted helix ferroelectrics (DHF), display an analog tilt of the cell optical axis in the plane of the cell walls upon application of an electric field across the smectic layers. In a discrete, multi-state cell, for example using ferroelectric SmC.sup.* or SmH.sup.* or antiferroelectric phases, application of an electric field above a certain threshold voltage results in switching of the tilt of the CSLC molecules between discrete stable states. In homeotropically aligned cells (smectic layers parallel to substrate walls), the optical axis of the CSLC material rotates in a plane perpendicular to the cell walls on application of an electric field across the smectic layers by electrodes that are lateral to the substrate walls.
Smectic liquid crystal cells have been employed in a wide range of device applications. Linear optical effect devices function by reorientation of the molecular director, a vector on the long axis of the molecule. Discrete and continuously tunable single and multiple stage color filters employing ferroelectric and chiral smectic liquid crystals are described by Johnson et al., U.S. Pat. No. 5,132,826. Intensity modulation is achieved by placing a smectic liquid crystal cell between polarizers (Lagerwall et al., U.S. Pat. No. 4,838,663). Phase, intensity and wavelength modulation by chiral smectic liquid crystal cells within Fabry-Perot cavities are described by Sharp et al. in U.S. patent application Ser. No. 07/792,284 filed Nov. 14, 1991.
In addition to the linear optical effects described above, second-order nonlinear optical effects in chiral smectic liquid crystals have been demonstrated, Chiral smectic liquid crystals display both second harmonic generation (Liu et al., Opt. Lett. 15, 267 (1990)) and the linear electrooptic effect (Liu et al., U.S. patent application Ser. No. 07/938,997 filed Sep. 2, 1992. However, the conversion efficiency for second-order nonlinear optical effects in liquid crystals is low and so a long interaction length is required.
In the rapidly growing field of optical fiber communications, devices such as phase and polarization controllers, amplitude switches, and multiplexer-demultiplexers are essential. Switch arrays, tunable filters, and high-speed modulators have been demonstrated using guided-wave devices fabricated with inorganic materials such as lithium niobate (Alferness, Science 234, 825 (1986)) and with organic polymers. Fiber optic polarizers have also been demonstrated (Bergh et al., Opt. Lett. 5, 479 (1980) and Dyott et al., Opt. Lett. 12, 287 (1987)).
Liquid crystals have been incorporated in devices used with optical fibers. Nematic liquid crystals provide coupling between guided light in two single mode fibers when a section of the fiber walls is ground down to the vicinity of the fiber core and a layer of nematic liquid crystal is placed between them (Goldburt et al., Appl. Phys. Lett. 46, 338 (1985)). Tunable wavelength nematic liquid crystal Fabry-Perot filters (Patel, U.S. Pat. No. 5,150,236) can be employed between fibers for wavelength demultiplexing. Liquid crystal Fabry-Perot filters have been mounted in a guided sleeve between two optical fibers (Hirabayashi et al., Jpn. J. Appl. Phys. 31, L1355 (1992)).
Waveguides have been fabricated with organic cores. A fiber was constructed with a polymethyl-methacrylate (PMMA) polymer core (Bosc et al., IEEE Photonics Tech. Lett. 4, 749 (1992)). Waveguide structures have been fabricated with a homeotropically aligned smectic liquid crystal core in which the molecular directors are perpendicular to the wall of the guided structure (Lo et al. Nonlinear Optics: Materials, Phenomena and Devices Digest, 37, IEEE Cat. No. 90CH2905-8 (1990)).